The basic image-forming process of color photography comprises the exposure of a silver halide photographic recording material to light, and the chemical processing of the material to reveal a useable image. The chemical processing entails two fundamental steps. The first is the treatment of the exposed silver halide with a color developer wherein some or all of the silver halide is reduced to metallic silver while an organic dye is formed from the oxidized color developer. The second is the removal of the silver metal thus formed and of any residual silver halide by the desilvering steps of 1) bleaching, wherein the developed silver is oxidized to silver salts; and 2) fixing, wherein the silver salts are dissolved and removed from the photographic material. The bleaching and fixing steps may be performed sequentially or as a single step.
It is highly desirable to process a photographic recording material as rapidly as possible, and an accelerated process is constantly being sought. In particular, shortening the silver removal step, which consumes almost half of the total process time, is an attractive way in which to shorten the overall processing time.
Juxtaposed to the desire for an accelerated process is the desire for, and the need for, photographic elements and process solutions that require lower chemical usage and that generate less polluting chemical waste. Used or spent fixer solutions are desilvered by electrochemical means to recover the semi-precious metal, but these solutions may still contain ingredients that limit their direct discharge into public waste water streams. Further treatment is required for them to conform to standards for environmentally acceptable disposal. In some photographic processes, higher processing speed can be traded for an indirect advantage in the form of lower processing chemistry usage and lower processing waste per unit of photographic material processed. Thus, these desires or goals are interrelated.
Numerous materials and methods suitable for the bleaching of metallic silver deposits have been described. Chief among the more useful bleaching or oxidizing agents are the higher oxidation states of various metals such as cerium, magnesium, chromium, vanadium, and iron, and the peracids, which include peroxide, persulfate, and their salts. The peracid bleaches may require the presence of a bleach accelerator to enable adequate bleaching. Likewise, many materials and methods suitable for the fixing or dissolution of silver salts and their removal from films are also known.
Commonly used fixing agents are the various salts of thiosulfate and thiocyanate, although other silver complexing anions have also been employed from time to time. This field of endeavor is generally reviewed in T. H. James, ed.,The Theory of the Photographic Process, Macmillan, New York, 1977, and in particular at Chapter 15, entitled "Complementary Processes", pages 437 to 461 of this text. Additional teachings are shown at Research Disclosure, December 1989, item 308119, published by Kenneth Mason Publications, Ltd., The Old Harbourmaster's, 8 North Street, Emsworth, Hampshire P010 7DD, England, and in particular, at Section XIX, entitled "Processing" of this publication.
For reasons related to cost and efficacy, the photographic processing industry has focused on the use of chelated iron bleaches and thiosulfate fixers for the desilvering of color photographic materials. It is well appreciated in the art that the benefits of thiosulfate fixers are enhanced in the presence of ammonium ion. Fixer solutions containing ammonium thiosulfate are more active and solubilize silver halide in a photographic element more rapidly than thiosulfate salts of other cations. Thus, owing to its rapid fixing action, ammonium thiosulfate is widely used as a photographic fixing agent.
In recent years, however, the need to provide diminished environmental impact from photographic processing solutions has led to increased interest in means of rendering photographic bleaches and fixers ecologically benign. A useful means for making photographic fixer solutions environmentally more acceptable, including those fixer solutions which employ thiosulfate anion as a fixing moiety, has been substituting sodium ion for ammonium ion as the cationic counterion in the fixer solution. The reduction or total removal of the more common ammonium ion from the processing solution enables reduced aquatic toxicity and reduced biological oxygen demand in wastewater streams. Use of sodium thiosulfate fixing agents has been disclosed, for example, in U.S. Pat. No. 2,195,405 and in the Theory of the Photoaraphic Process and Research Disclosure cited above.
This substitution of sodium ion for ammonium ion is, however, commercially undesirable since sodium thiosulfate tends to be a poorer fixing agent than ammonium thiosulfate and thus requires either higher concentrations of fixing agent or longer fixing times to enable adequate silver salt dissolution. Various solutions to this problem have been disclosed. Specific processes utilizing color photographic recording materials comprising tabular grain emulsions with sodium thiosulfate fixer solutions to enable improved fixing are disclosed in co-pending U.S. patent application Ser. No. 07/747,895 entitled "Color Photographic Recording Material Processing" filed Aug. 19, 1991. Other attempts have been made to shorten the fixing process using various fixing accelerators.
Many combinations of sequential bleaching and fixer solutions and unitary bleach-fix solutions have been disclosed over the years. Typical examples are shown in the Research Disclosure and Theory of the Photographic Process cited above. With the current emphasis on ecologically sound processing, processes which utilize peracid bleaches are especially preferred since these bleaches tend to have a low environmental impact. A specific process utilizing a photographic recording material comprising high tabularity tabular grain emulsions in combination with peracid bleaches is disclosed in co-pending U.S. patent application Ser. No. 07/891,601 entitled "Method of Processing A Photographic Element With A Peracid Bleach," filed June 1, 1992.
It is known in the art that films with a high iodide content are more difficult to fix as discussed in British Patent 1,476,330 (p. 1, lines 50-58) and U.S. Pat. No. 4,960,683 (col. 16, line 18-28) Therefore, lowering the iodide content of a silver halide photographic element will increase its rate of fixing. It is often desirable, however, to have a higher iodide content in a photographic element, particularly in color negative systems. The high iodide content can enable improved blue light absorption and improved blue speed. High iodide phases in individual grains are also said to enable improved conversion of absorbed light into latent image thus further improving sensitivity.
A process which includes the ecological advantages of both a peracid bleach and a sodium thiosulfate fix is highly desirable; however, such a process has been thought to sacrifice speed in desilvering. This is especially true for the high iodide photographic elements. Photographic processes utilizing both a persulfate bleach solution, a species of peracid bleach, and a sodium thiosulfate fixer solution are incidentally disclosed as examples in U.S. Pat. Nos. 4,448,878; 4,481,290; 4,458,010; 4,506,007 and 4,508,816. These publications are directed at bleaching improvements and disclose numerous combinations of different bleaches with various fixers.
The sodium thiosulfate fixer solutions, incidentally illustrated in these publications, are used at high concentrations and long fixing times so as to ensure adequate fixing. These are exactly the fixing conditions that would be anticipated as useful based on the known inferiority of sodium thiosulfate as a fixitive agent. No mention is made of any differences in the fixing behavior of photographic recording materials as a result of the composition of the bleaching agent employed. There is no suggestion of any fixing benefit to be drawn from a photographic process combining any specific bleaching solution and a fixer solution comprising sodium thiosulfate. Further, the process described is utilized with low iodide photographic elements. There is no indication that such a process is particularly useful with high iodide elements.
There remains a need for an environmentally sound method of desilvering developed photographic images, particularly those with a high iodide content, without sacrificing the speed at which these images can be provided to a customer.